Securities - Maturity of Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Held for Investment, Contractual maturity of debt securities, Amortized Cost
Amortized cost, Within one year	$ 975	$ 1,036
Amortized cost, After one year but within five years	428	623
Amortized cost, After five years but within ten years	599	625
Amortized cost, After ten years	6	8
Amortized Cost	2,008	2,292
Available for sale, Contractual maturity of debt securities, Amortized Cost
Amortized Cost, Within one year	114,086	113,225
Amortized Cost, After one year but within five years	296,057	289,038
Amortized Cost, After five years but within ten years	6,129	6,222
Amortized Cost, After ten years	49,472	55,813
Total debt securities	465,744	464,298
Amortized Cost, Equity securities		6,527
Amortized Cost	465,744	470,825
Held for Investment, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	978	1,034
Estimated Fair Value, After one year but within five years	428	627
Estimated Fair Value, After five years but within ten years	605	633
Estimated Fair Value, After ten years	6	9
Estimated Fair Value	2,017	2,303
Available for sale, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	113,828	112,974
Estimated Fair Value, After one year but within five years	291,843	287,058
Estimated Fair Value, After five years but within ten years	6,306	6,500
Estimated Fair Value, After ten years	48,886	55,467
Total debt securities	460,863	461,999
Estimated Fair Value, Equity securities		5,704
Estimated Fair Value, Total	$ 460,863	$ 467,703